Even he did knew about it, Orochimaru sees more potential about the sharingan, itīs his personal belief that a ninjaīs strenght is based in how many jutsu a single warrior has. Iīm pretty sure that to him, the ability to know every technique there is is better than Rinnegeanīs multi-element affinity.

The very fact that the Rinnegan allows the usage of all five elements seems like a very good reason for Oro to consider it over the Sharingan, since having all affinities is necessary for having all jutsus.

May those who accept their fate be granted happiness; to those who defy it, glory.